This invention relates to an apparatus for measuring water vapor concentrations in a gas; or particularly the invention relates to the construction of a measurement cell for a water vapor sensor.
Instruments for measuring minute quantities of water vapor in a gas are well developed in the prior art. For example, U.S. Pat. No. 3,174,037, issued Mar. 16, 1965, discloses an apparatus and method for measuring the concentration of a gas in a mixture of gases, wherein the preferred embodiment relates to the measurement of water vapor in air. U.S. Pat. No. 3,902,068, issued Aug. 6, 1975, and owned by the assignee of the present invention, discloses a method and apparatus for measuring the quantity of a test gas such as butane which is present in an absorption cell. This patent relates to an apparatus for measuring the permeability of a membrane which is positioned to isolate the test gas cell from the absorption cell.
U.S. Pat. No. 5,390,536, issued Feb. 21, 1995, and owned by the assignee of the present invention, discloses a system for measuring water vapor permeability through a membrane, and utilizing the general principles of the '068 patent to measure the water vapor concentrations in an absorption cell.
All of the aforementioned patents utilize an infrared radiation source to generate radiation through windows into the absorption cell and the amount of this radiation which passes through the cell is monitored by an appropriate detector. U.S. Pat. No. 3,902,068 further utilizes a pumping apparatus for subjecting the gas in the absorption cell to pressure pulsations, thereby alternately increasing and decreasing the gas density in the absorption cell. The radiant energy passing through the cell is affected by the relative pressure fluctuations, leading to an output radiation signal which can be translated into an electrical alternating current output signal proportional to the gas concentration in the cell.
One of the problems in using an absorption cell of the aforementioned type is caused by undesired radiation signals which may result from heating effects of the absorption cell and/or internal reflections of the radiant energy within the cell. Therefore, the radiant energy signal which is passed through the cell via the windows in the cell includes a desired radiation signal plus an additional radiation signal which can be attributable to "noise" caused by the foregoing and perhaps other effects. In the prior art the measured water vapor concentrations were quite high, and therefore desired signal strength is sufficiently large so as to permit the noise component of the radiant energy signal to be filtered, while preserving an adequate amplitude of the desired signal. However, this limits the linearity of the instrument at water vapor permeation levels below about 10 grams per square meter per day (10 gm/M.sup.2 /day), and requires flow rate and other adjustments at extremely low levels of water vapor permeation; i.e., at levels below about 1 gm/M.sup.2 /day. Water vapor permeation below this level were inherently difficult, if not impossible, to measure. Extremely low levels of water vapor concentration will arise in an absorption cell when an instrument of the type described is used to test permeability of membranes having an inherent low water transmissivity characteristic. For example, films having a transmissivity of water vapor down to the range of approximately 10 grams per square meter per day (gm/M.sup.2 /day) can be readily measured in absorption cells of the type disclosed in the '068 patent without regard to the radiation noise component of the signal. However, recent technology advances in the manufacture of films such as coated films, produce transmissivity rates down to the range of under 0.01 gm/M.sup.2 /day, and such readings are severely affected by the aforementioned radiation noise components.
A significant problem which has been noted in attempting to use prior art absorption cells for measurements of extremely low water vapor permeation levels is the problem of nonlinearity. It has been noted that the measured radiation signal becomes nonlinear at 5 to 6 gm/M.sup.2 /day permeability measurements, and the usability of the instrument has thereby been diminished. The reasons for this nonlinear behavior are not fully understood and empirical testing has shown that the nonlinearity characteristic varies to some extent from instrument to instrument. These problems limit the usefulness of the prior art instruments for measuring some newly developed films.